Global disease burden data from UNICEF and WHO show that the top two major causes of infant and child mortality are respiratory and diarrheal diseases. While diarrheal diseases caused by enteric pathogens constitute one of the major causes of morbidity and mortality globally, obtaining accurate epidemiological information about the causative agents is essentially impossible. WHO estimates are just that and the apportionment of disease due to bacteria versus viruses versus parasites are often educated guesses at best and are further complicated due to infections with more than one causative pathogen. Enteric fever due to Salmonella Typhi and S. Paratyphi A & B is still a major worldwide problem affecting at least 20-30 million people with a significant mortality in medically underprivileged countries. Experts on Shigella infections estimate 165 million annual episodes of infection with 1.1 million deaths. If one collects all these data and the information of diarrheal diseases due to E. coli pathovars, C. perfringens type A, C. jejuni plus the enteric viruses and parasites, the total annual mortality due to diarrheal diseases approaches 5 million. Guerrant et al., Arch. Med. Res. 33:351-355 (2002) estimate that there are 3 million annual deaths from diarrheal diseases and thus might be closer to the truth. The WHO estimate that children in Africa experience an average of five diarrheal episodes per year with 800,000 deaths due to fluid loss and dehydration nevertheless indicates the magnitude of the problem. The results of a 4-year WHO study found that between 2000 and 2003, diarrheal diseases accounted for 18% of the 10.6 million annual deaths in children under 5 years of age.
The problem is exacerbated by cessation in use of subtherapeutic antibiotic additions to animal feed for growth promotion. This was first recognized in poultry and other livestock such as swine but C. perfringens also causes necrotizing enteritis in the small intestines of humans, which occurs sporadically in underdeveloped countries. Some factors that predispose to C. perfringens induced necrotic enteritis include protozoan and helminth infections. C. perfringens type associated diarrhea is one of the top 5 causes of food borne bacterial diarrheal disease ranked by CDC in the U.S. Alpha-toxin is particularly responsible for sublethal effects on enterocytes that could lead to malabsorption and stunting in children in developing countries. Studies also show the possible etiologic significance of early intestinal C. perfringens colonization and development of necrotizing enterocolitis in newborns. Yersinia enterocolitica and Y. pseudotuberculosis are other intestinal colonizers that may contribute to human diarrheal disease, but these enteropathogens have not been well studied as contributors to intestinal disease of humans in the developing world.
The problems of travelers in acquiring diarrheal diseases when abroad are legend and often ascribed to ETEC and EPEC strains but just as likely might be due to Norwalk and rotaviruses, Campylobacter, Listeria or Giardia, all predominantly waterborne or foodborne infections. Although enteric pathogens have a much larger detrimental effect on health in the developing world, these infections are not without major economic consequences in the U.S. and other developed countries. The USDA estimated that infections with Campylobacter, Salmonella (non-typhoidal), EHEC, STEC and Listeria had a 6.9 billion dollar negative economic impact. E. coli is the leading cause of both community-acquired and nosocomial urinary tract infections (UTI). As many as 50% of women have had at least one episode of UTI in their lifetime. E. coli also causes 12-50% of nosocomial infections. In regard to bacterial enteropathogens, a major problem, except for host-adapted Salmonella (i.e., S. Typhi and S. Paratyphi), Shigella sp. and some ETEC and EPEC strains, is the vast animal reservoir of Salmonella enterica serotypes (over 2000), Clostridium sp., Yersinia sp., APEC and other E. coli pathovars, Listeria and Campylobacter. Animals including companion animals, wildlife, and agriculturally important food animals are causes of water contamination or transmission of bacterial enteric pathogens through the food chain to humans. As such, a vaccine that reduces the probability of infection by necessitating higher infection doses or a vaccine that lessens the consequences of infection offers a definite public health benefit, especially in the developing world.
Approximately 1.4 million humans are infected with Salmonella enterica serotypes each year in the U.S. primarily causing gastroenteritis and lost time from work, but with a low incidence of more severe infections, sometimes leading to death in the very young, the elderly or in individuals with an immunocompromising condition, such as advanced HIV infection. In the U.S., Salmonella accounts for 31% of the fatalities due to foodborne pathogens whereas Listeria monocytogenes accounts for 28% and C. jejuni, which causes many more infections (2.5 million), accounts for 1% of the deaths. Twenty percent of all Salmonella cases or isolates are from children under 5 years of age.
Salmonella enterica has been subdivided into seven subspecies differentiated by biochemical and genetic tests, with subspecies I containing most of the serotypes that are implicated in warm-blooded animal and human infection. Although data collected from year to year and from country to country differ, it would appear that poultry (contaminated eggs and meat) constitute the major source of food-borne Salmonella infection in humans with contaminated pork, dairy products and vegetable/fruit crops accounting for the rest, but in decreasing frequency of causation. In a recent study, the Food Safety and Inspection Service (FSIS) determined Salmonella serotypes isolated from swine, ground turkey, ground beef and broilers in processing plants participating in the Hazard Analysis and Critical Control Point (HACCP) systems for pathogen reduction and found that 87% of the Salmonella isolates were from poultry sources. Using data from the Centers for Disease Control and Prevention collected in 2005, it is evident that some Salmonella serotypes that are most frequently isolated from humans are also very prevalent in poultry, with 8 of the Salmonella serotypes predominantly isolated from poultry being represented in the top 20 serotypes isolated from humans.
Salmonella is a gram-negative bacterium, best known for causing enteric diseases. Within the Salmonella genus, there are two main species, S. bongori and S. enterica. However, within each species, there are over 2500 serovars. These numerous serovars are found in a wide variety of different environments and are associated with many different diseases. The vast majority of human isolates (>99.5%) are subspecies S. enterica. To simplify taxonomy, the Centers for Disease Control and Prevention recommend that Salmonella species be referred to only by their genus and serovar, e.g., Salmonella Typhi (or S. Typhi) instead of the more technically correct designation, Salmonella enterica subspecies enterica serovar Typhi.
One important use of genetically engineered microorganisms, such as Salmonella, is as a live vaccine for inducing immunity. The use of Salmonella for vaccine purposes requires that the Salmonella be attenuated such that the administration thereof does not induce disease symptoms associated with wild type Salmonella infection. In addition, the Salmonella vaccine also has to exhibit a high degree of immunogenicity. As such, the objective of much research on Salmonella-based vaccines is to construct a safe and efficacious Salmonella vector system that can be used repeatedly for multiple recombinant attenuated Salmonella vaccines (RASVs) and additionally, induce some level of cross-protective immunity to diarrheal diseases caused by the diverse S. enterica serotypes and other pathogenic enteric bacteria (e.g., Shigella sp. and E. coli pathovars).
In theory, the ideal attenuated Salmonella vaccine should exhibit wild-type abilities that are capable of withstanding all types of biological stress that is entailed with living in an individual. Examples of these types of biological stresses include: exposure to enzymes, acid, bile, osmotic pressures and ion stress. In addition, the ideal attenuated Salmonella vaccine should also be able to withstand host defenses encountered following administration (e.g., orally or intranasally). Further, the ideal attenuated Salmonella vaccine should also be able to colonize and invade host lymphoid tissues before displaying its attenuation and its inability to cause disease symptoms.
Another existing problem is that the recipient's immune system reacts to the Salmonella serotype-specific antigen. The combination of minimizing a recipient's immune response to the Salmonella serotype-specific antigen while maximizing the immune response against the undesired bacterial pathogens has not been effectively accomplished in the art. What is needed are compositions and/or vaccines of Salmonella that are capable of decreased or absent expression of Salmonella serotype-specific antigens that can elicit immune responses to antigens expressed by bacterial enteric pathogens.
All references, patents, and patent applications cited here are each incorporated by reference in their entirety for all purposes.